Presently, much attention and developmental effort is being focused on digital mobile radio telephone systems. The mobile stations and the respective base stations of a TDMA system communicate across time division multiple access channels whereby multiple mobile stations share the same transmission and reception frequencies, but are assigned separate timeslots within those frequencies. In normal operation, the mobile stations transmit bursts at certain times allowing for propagation delays so that the bursts are received at a base station in their assigned timeslots. Otherwise, transmissions from different mobile stations collide or overlap, resulting in mutual interference in reception at the base station. Typically, a guard band is inserted between the timeslots to help prevent the bursts from overlapping. However, the guard bands are generally only a few symbols wide and as a result only guard against small time alignment errors.
In the Global System for Mobile Communications (GSM), the mobile station is effectively synchronized to the downlink from the base station and therefore the "local" time at the mobile station is effectively always somewhat slow compared to the base station because of the time taken by the propagation of the downlink signal. Uplink bursts are further delayed because of the finite speed of light. Therefore, uplink signals from mobile stations at different distances will arrive with varying degrees of lateness unless compensating measures are taken.
In GSM, the base station is able to measure of degree of "lateness" of a received uplink burst, and using a parameter known as timing advance value, is able to instruct the mobile to transmit its burst earlier, such that allowing for the propagation delay for the uplink signal, the burst is received at the base station at exactly the right time. In other words, the burst is actually transmitted by the mobile station in advance of its nominal timing.
Mobile stations at different distances to the base station therefore transmit their uplink bursts at different times based on different timing offsets, but the end result is that each burst is received in its nominal timeslot, and thus the bursts do not interfere with each other. The timing advance parameter is periodically resent to the mobile station, and therefore changes in the propagation delays due to the movement of the mobile station can be compensated for before problems arise.
However, in most cellular systems, the timing advance has a maximum value which corresponds to a maximum cell radius. For example, in GSM, the timing advance is a 6 bit variable value ranging from 0 to 63. The limited range of the timing advance value effectively restricts the maximum size of the cell. When the timing advance has a value of 63, the cell radius is approximately 35 kilometers in GSM. If the mobile station exceeds this radius, the burst can not be advanced any further and as a result, the burst received at the base station will start to slide back into the next timeslot corrupting both bursts.
One method for extending the range of a cell in a TDMA system is to assign a mobile station two consecutive timeslots rather than one timeslot as illustrated in FIG. 1. In FIG. 1, an 8-timeslot frame has been divided into 4 groups 10, 12, 14, and 16 which each contain two consecutive timeslots. In FIG. 1, the "white" inner box 18 illustrates the nominal position of a burst, which has slid backward while its actual arrival time is illustrated by the shaded boxes. By assigning each mobile station two consecutive timeslots and assigning the nominal position of the burst to the first of the two timeslots, a mobile station can travel past the original cell radius since the burst can slide back into the second timeslot without interfering with the burst of another mobile station. As the distance between the mobile station and the base station increases, the burst will slide back further and further into the second timeslot. A new limit is reached when the burst slides back an entire timeslot. In the GSM system, this new limit corresponds to approximately 100 km. If the mobile station goes any further from the base station, the burst will then start to slide into the next, i.e., third, timeslot which may corrupt a burst in the following timeslot.
The disadvantage in using this prior art solution to extend the range of each cell in a system is that the capacity of the system is effectively halved since each mobile station is assigned two timeslots rather than one. Thus, there is a need in the art for a method of extending the range of a TDMA system without drastically reducing the capacity of the TDMA system.
In prior systems, a mobile station is assigned to an available timeslot regardless of the propagation delay of the mobile station. This is illustrated in FIG. 1, where the burst in window 2 is seen to have slid less than the burst in window 1.